Field
This invention relates generally to a strain-gauged washer and, more particularly, to a conventional-thickness washer for measuring bolt preload including one or more strain gauges placed in a slot in the washer and oriented through a thickness of the washer to measure compressive strain in the washer, where the slot is located between an inner diameter and an outer diameter of the washer in order for the strain gauge to detect a maximum strain field, and more than one strain gauge may be connected in series in order to increase effective strain gauge length.
Discussion
Bolts are commonly used to fasten components together in assemblies of all types—ranging from simple, inexpensive household items to multi-billion dollar aircraft and space vehicles. Most bolted joints include a washer under the bolt head—where the washer serves to provide uniform contact and prevent damage to the underlying component.
In many bolted joint applications, it is important to achieve a prescribed preload in the bolt. Proper bolt preloading is effective in minimizing joint fatigue due to cyclic loading, and is also effective in preventing bolt loosening or back-out. Bolt preloading requirements are especially important in applications where the article of manufacture is large (requiring significant disassembly in order to access and replace/tighten a bolt), expensive (costly downtime for bolt replacement/tightening) and/or remotely deployed (impossible to replace/tighten a bolt on a satellite in space).
Many techniques for determining bolt preload have been developed over the years. One of the most basic forms of bolt preload estimation is through simple torque measurement during bolt tightening. However, surface friction and thread friction variations make torque-based bolt preload estimation inherently imprecise—with accuracies often no better than +/−30%. Other bolt preload techniques involve instrumentation or inspection of the bolt itself. These techniques also have disadvantages, however, including the cost and complexity of fitting sensors inside of the bolt, and the time and labor involved in performing ultrasound or other inspections on every bolt after it is installed. Still other bolt preload techniques involve the use of a thick collar in place of a standard washer under the bolt head, where the collar is fitted with instruments for measuring or estimating the load applied by the bolt head. However, these thick instrumented collars change the geometry of the bolted joint, necessitating a different bolt length to be used and/or dimensional changes to the fastened components.
As discussed above, all of the traditional techniques for bolt preload estimation or measurement suffer from significant drawbacks. Therefore, a need remains for a bolt preload measurement technique which is simple, inexpensive, reliable, accurate, and does not require any changes to the bolts or fastened components which are used in a bolted assembly.